Cyclohexane oxidation on a novel Ti70Zr10Co 20 catalyst containing quasicrystal

Article abstract of DOI:10.1039/b905324g

Ti₇₀Zr₁₀Co₂₀ containing an icosahedral quasicrystalline phase has been fabricated, and presents high activity and selectivity in catalyzing the oxidation of cyclohexane with oxygen under solvent-free conditions.

Full text of DOI:10.1039/b905324g

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Cyclohexane oxidation on a novel Ti Zr Co catalyst containing
10
70
20
quasicrystalw
Jianmin Hao, Baozhong Liu, Haiyang Cheng, Qiang Wang,ab Jinyao Wang,ab
ab
c
ab
a
a
Shuxia Cai and Fengyu Zhao*
Received (in Cambridge, UK) 17th March 2009, Accepted 3rd April 2009
First published as an Advance Article on the web 6th May 2009
DOI: 10.1039/b905324g
Ti70Zr10Co20 containing an icosahedral quasicrystalline phase
has been fabricated, and presents high activity and selectivity in
catalyzing the oxidation of cyclohexane with oxygen under
solvent-free conditions.
(five-fold), which indicates an unusual atomic structure with
1
2
long-range ordering. Because of their unique structure and
special physical and/or chemical properties, quasicrystals are
expected to be attractive catalytic materials. It has been
reported that an Al–Cu–Fe quasicrystal exhibited high activity
1
3
The selective oxidation of cyclohexane is of increasing
importance for the modern chemical industry, because its
products (cyclohexanol and cyclohexanone) are starting
materials for the synthesis of adipic acid and caprolactam,
which are intermediates in the manufacture of nylon-6 and
for steam reforming of methanol. Herein, a Ti70Zr10Co20
catalyst containing a quasicrystalline phase was fabricated
by the arc-melting method, and its catalytic performance is
discussed in cyclohexane oxidation with molecular oxygen
under solvent-free conditions. The Ti70Zr10Co20 catalyst
samples were characterized by XRD, ESEM, EDX, TEM,
SAED and ICP-OES. The results showed that it is a novel
and efficient catalyst for cyclohexane oxidation with a
conversion above 6.0% and selectivity of cyclohexanol
and cyclohexanone higher than 90%. In particular, the
Ti70Zr10Co20 catalyst could be recycled several times without
loss of activity or selectivity.
1
nylon-66 polymers. Industrially cyclohexane oxidation is
generally catalyzed by soluble cobalt metal salts or complexes
at temperature above 423 K, in which the total conversion
of cyclohexane is lower than 4% and the selectivity of
2
cyclohexanol and cyclohexanone is around 70–85%.
Increased environmental concerns call for benign oxidation
including heterogeneous catalysts, solvent-free reaction and
clean oxidants. Molecular oxygen as oxidant has attracted
much more attention because it is readily available and
Fig. 1 shows the XRD patterns of the Ti70Zr10Co20 catalyst
samples. The diffraction peaks of fresh Ti70Zr10Co20 catalyst
correspond to the I-phase and b-(Ti,Zr) phase, and the I-phase
is indexed following the scheme originally proposed by Bancel
3
,4
environmentally benign compared with other oxidants.
Recently, several heterogeneous catalysts including metal-
containing redox molecular sieves, oxides, and nanoparticles
have been developed for the oxidation of cyclohexane, and
most of these heterogeneous catalysts are designed with
1
4
et al. The diffraction peaks of Ti70Zr10Co20 catalyst after
being reused five times show the same I-phase and b-(Ti,Zr)
phase as the fresh catalyst, as well as a new broad strong
peak at 2y around 22.81, which was indexed principally as
hexagonal carbon from the recognized XRD data, which is
consistent with the results of ESEM as shown in Fig. 2(d). The
5
–8
cobalt as the active species.
Moreover, the mechanism
of cyclohexane oxidation has been discussed by Hermans
9
et al. They reported that the pronounced chain-initiation
enhancement by the ketone is attributable to a concerted
reaction between cyclohexyl hydroperoxide and cyclohexanone,
and the abstraction of the weakly bonded aH-atom of the
primary hydroperoxide product by chain carrying peroxyl
radicals is the source of major end products such as alcohol
and ketone/aldehyde.
Since quasicrystals were discovered in 1984, a number of
icosahedral quasicrystalline phases (I-phase) such as Ti-based
and Al-based phases have been fabricated successfully in
1
0,11
various systems.
I-Phases show a sharp diffraction pattern
corresponding to traditionally forbidden rotational symmetry
a
State Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, P.R. China. E-mail: zhaofy@ciac.jl.cn;
Fax: +86-431-85262410; Tel: +86-431-85262410
Graduate School of the Chinese Academy of Sciences, Beijing,
100049, P.R. China
b
c
Institute of Material Science and Engineering, Henan Polytechenical
University, Jiaozuo, 454000, P.R. China
Fig. 1 XRD patterns of Ti70Zr10Co20 catalyst samples: (a) fresh
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b905324g
catalyst; (b) catalyst reused five times.
3
460 | Chem. Commun., 2009, 3460–3462
This journal is ꢀc The Royal Society of Chemistry 2009

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quasicrystal is more brittle. Furthermore, the size of catalyst
particles did not change after the fifth run from that shown in
the results in Fig. 2(a) and (c). However a black shadow was
observed on the surface of the catalyst after it was reused five
times (Fig. 2(d)); this is carbon deposition, confirmed by EDX
analysis. It is supposed that some hydrocarbons are strongly
adsorbed on the surface of the catalyst, which could lead to the
formation of carbon deposition by dehydrogenation and
fragmentation from further oxidation at high temperature.
The back-scattering SEM micrograph of Ti70Zr10Co20 catalyst
particles in Fig. 2(b) shows that it consists of two distinct
phases, namely, an irregular gray area and a dark matrix.
EDX analysis shows that the dark matrix mainly contains Ti
and Zr, and it is identified as a b-(Ti,Zr) solid solution phase;
the gray area corresponds to an I-phase with a composition of
Fig. 2 ESEM images of Ti70Zr10Co20 catalyst samples: (a) fresh
approximately Ti Zr Co .
61 11 28
catalyst; (b) back-scattering SEM micrograph of fresh catalyst;
(
c) catalyst after the fifth run; (d) selected piece of catalyst particle
Fig. 3 illustrates a TEM bright-field image and selected
electron diffraction patterns of a Ti70Zr10Co20 catalyst sample.
The selected electron diffraction patterns are identified as the
[110] orientation of b-(Ti,Zr) and a two-fold symmetry of
the I-phase. Since the first report of I-phase formation in a
after the fifth run.
ESEM images of Ti70Zr10Co20 catalyst samples showed
some cracks in the catalyst particles in Fig. 2(a) and (d) which
were formed through the alloy crushing process since the
1
5
rapidly quenched Ti53Zr27Co20 alloy, few examples have
been reported of I-phase formation in Ti–Zr–Co alloys. To
the best of our knowledge it is the first time an I-phase has
been obtained in a Ti–Zr–Co cast ingot sample.
The catalytic performances of Ti–Zr–Co catalysts of
different compositions have been discussed and compared
with several cobalt catalysts, and the results are given in
Table 1. The oxidation of cyclohexane was carried out under
2
the optimum conditions of 413 K and 2 MPa O as reported in
1
6
our previous work with Ti–Zr–Ni–Cu catalysts. It was found
that with an increase in the Co content from 5% to 40%, the
conversion increased initially from 2.0% to 6.8% and then
decreased to 4.3%. The selectivity of A + K + CHHP
decreased linearly. It is suggested the alloy composition has
a crucial influence on its activity: with increasing Co content,
the reaction rate increased. However a higher Co content in
Ti–Zr–Co alloy catalysts could accelerate the oxidation of A
Fig. 3 TEM bright-field image and selected electron diffraction
patterns of a Ti70Zr10Co20 catalyst sample.
a
Table 1 Catalytic oxidation of cyclohexane over Ti–Zr–Co catalysts
Selectivity (%)
Entry
Catalyst
t/h
Conversion (%)
A
K
CHHP
A + K + CHHP
1
2
3
4
5
6
7
8
6
6
6
6
6
6
4
8
6
8
6
6
0.9
2.0
2.6
6.8
4.6
4.3
5.1
7.0
5.7
5.8
2.8
7.6
33.3
46.2
49.1
40.2
20.5
21.7
51.6
37.5
45.7
36.5
51.1
39.6
33.6
34.0
33.4
50.2
56.4
51.8
44.5
44.9
48.7
41.2
33.3
49.5
27.9
19.4
17.0
—
—
—
—
—
—
94.8
99.6
99.5
90.4
76.9
73.5
96.1
82.4
94.4
91.9
84.4
89.9
Ti85Zr10Co
5
Ti80Zr10Co10
Ti70Zr10Co20
Ti60Zr10Co30
Ti50Zr10Co40
Ti70Zr10Co20
Ti70Zr10Co20
Ti70Zr10Co20
CoAlPO-5
b
9
c
1
1
1
0
1
2
14.2
—
0.8
d
d
Co
Co
3
O
O
4
3
4
a
Reaction conditions (entries 1–9): cyclohexane 8 ml, catalyst 40 mg, oxygen 2 MPa, temperature 413 K. A: cyclohexanol, K: cyclohexanone,
CHHP: cyclohexyl hydroperoxide. By-products are acids, esters, alcohols and ketones analyzed by GC-MS. The conversion was calculated by
dividing moles of products formed by initial moles of cyclohexane used, and the selectivity was calculated by dividing moles of a particular product
b
by the total moles of products formed. The results of recycling five times. The reaction was carried out at 403 K, 1.5 MPa air, in the presence of
d
TBHP in ref. 6. The reaction was carried out at 393 K, 1.0 MPa O
c
2
, in the absence (entry 11) and presence (entry 12) of TBHP in ref. 8.
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and K absorbed on the surface of the catalyst and produce
by-products, which congregate around the catalyst hindering
the adsorption of reactants on the catalyst surface, and thus
retarding the reaction rate and decreasing the selectivity. The
conversion did not change much with a value around 6% after
five runs (entry 9), and the selectivity of A and K tends to
increase from 90.4% to 94.4% in the recycling runs. The
formation of carbon deposited over the catalyst surface might
cover the catalytic active sites, and so the further oxidation of
A and K decreased inducing a small increase of the selectivity
in the recycling runs. The relation between carbon deposition
and catalytic activity and selectivity of metal oxide catalysts
The authors gratefully acknowledge the financial support
from the One Hundred Talent Program of CAS. Thanks are
also due to Prof. Limin Wang of the State Key Laboratory of
Rare Earth Resources Utilization, Changchun Institute of
Applied Chemistry, CAS, for helpful discussions.
Notes and references
1 D. D. Davis, in Ullmann’s Encyclopedia of Industrial Chemistry,
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2
K. Weissermel and H. J. Horpe, Ind. Org. Chem., VCH Press,
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1
7
3 Z. Hou, B. Han, L. Gao, Z. Liu and G. Yang, Green Chem., 2002,
, 426.
has been discussed in the literature by Hettige et al. In
comparison with heterogeneous catalysts like CoAlPO-5 and
Co O reported in the literature (entries 10–12), comparable
4
R. Zhao, D. Ji, G. Lv, G. Qian, L. Yan, X. Wang and J. Suo,
4
3
4
Chem. Commun., 2004, (7), 904.
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P. A. Jacobs, Angew. Chem., Int. Ed. Engl., 1995, 34, 560.
reaction conversion and selectivity were obtained with the
present Ti Zr Co catalyst containing an icosahedral
7
0
10
20
6
R. Raja, G. Sankar and J. M. Thomas, J. Am. Chem. Soc., 1999,
21, 11926.
7 V. Kesavan, P. S. Sivanand, S. Chandrasekaran, Y. Koltypin and
quasicrystalline phase without any additional free-radical
initiator. In addition, no metallic ions like Ti, Zr and Co were
detected in the filtered product liquid by ICP-OES analysis,
indicating that the present reaction is truly performed over the
catalyst surface heterogeneously. Therefore, the Ti70Zr10Co20
alloy with quasicrystalline properties is an efficient and
recyclable heterogeneous catalyst for cyclohexane oxidation.
1
A. Gedanken, Angew. Chem., Int. Ed., 1999, 38, 3521.
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J. Peeters, Chem.–Eur. J., 2006, 12, 4229; (c) I. Hermans, P. Jacobs
and J. Peeters, Chem.–Eur. J., 2007, 13, 754; (d) I. Hermans,
J. Peeters and P. A. Jacobs, Top. Catal., 2008, 50, 124.
In summary, a Ti Zr Co catalyst containing I-phase was
7
0
10
20
fabricated and was used for the first time as catalyst in the
oxidation of cyclohexane. The Ti Zr Co catalyst shows a
10 K. F. Kelton, P. C. Gibbons and P. N. Sabes, Phys. Rev. B, 1988,
3
8, 7810.
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7
0
10
20
1
1
1
1
fascinating and promising catalytic performance in the
oxidation of cyclohexane, and it could be reused several times
without loss of activity or selectivity. It should be noted up to
now that studies on catalysis of quasicrystals are very few, and
the catalytic mechanism as well as the relationship between the
catalytic performance and the microstructure is still unknown.
This kind of metal alloy with an icosahedral quasicrystalline
phase will attract more attention as a catalyst. We are
continuing to study further the catalytic performance of
quasicrystalline catalysts.
5
1
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3
462 | Chem. Commun., 2009, 3460–3462
This journal is ꢀc The Royal Society of Chemistry 2009